Prior to attaching a head gimbal assembly (HGA) into a hard disc drive, it is desirable to dynamically test the functionality of the read and write transducers that reside on the head gimbal assembly so that defective HGAs may be identified and sorted. Such testing can include preliminary activities to align, configure, and prepare the HGA for testing, followed by the actual electrical test of the HGA. Because HGAs are typically small, fragile, and contain sensitive electronic components, they are susceptible to mechanical stress, electro-static discharge (ESD), environmental contamination, and other handling-related issues.
To avoid these handling-related issues, current systems mount the HGA on an intermediate mounting fixture that supports the HGA throughout the testing process. An operator may manually place an HGA into an alignment tool that sets the orientation of the HGA to an intermediate mounting fixture. The alignment of the HGA to the intermediate mounting fixture is important because it helps determine the orientation of the HGA with respect to a disc during dynamic electrical testing. After alignment, a head set operation is performed in which the HGA and the intermediate mounting fixture are manually passed through a magnetic field to properly set the direction of the magnetic domains of the read and write transducers inside the head of the HGA.
Initially, the HGA's read and write transducers are electrically shorted together with a shunt tab, which resides on a flex circuit of the HGA and protects the HGA from ESD damage by ensuring that the components are held at a common voltage potential. This shunt tab must be broken or removed prior to testing the HGA. In current systems, the shunt tab is manually broken or cut off before the HGA is loaded into the electrical tester. After its removal, the HGA becomes extremely sensitive to ESD damage. Positioning the flex circuit for removal of the shunt tab is challenging because the flex circuit is flexible, and its position can vary over a relatively wide area. Additionally, flex circuits may have an inherent bend or twist, further complicating flex circuit positioning. In current systems, the intermediate mounting fixtures have positioning pins to precisely locate the flex circuit for de-shunting.
When the HGA is ready for electrical test, an operator can manually pick the HGA from a tray by grasping the intermediate mounting fixture and loading the HGA onto a dynamic electrical tester. During the dynamic electrical test procedure, the HGA's flex circuit makes interconnect with the dynamic electrical tester's preamplifier, the HGA is loaded to a test disc, and the read and write transducers on the HGA are tested. Using this method of HGA manipulation and electrical testing requires a new intermediate mounting fixture to be designed and fabricated for each new HGA type. The intermediate mounting fixture generally consists of a clamping mechanism to hold the HGA base plate, a set of pins to locate the HGA flex circuit for interconnect, and a set of holes and pins to locate the intermediate mounting fixture at the various operations, including dynamic electrical test.
The clamping mechanism of the intermediate mounting fixture that holds the HGA base plate during electrical test has several requirements. As the bit density in disc drives increases, in operation the drive heads must fly lower with respect to the disc. This requires tighter tolerances for the HGA's orientation. Errors in the orientation influence Roll Static Attitude (RSA) and Pitch Static Attitude (PSA), which affect the HGA's ability to load to the disc and its fly characteristics after loading. RSA and PSA are effectively the head's static orientation relative to the disc. To ensure that the HGA's performance is consistent for both electrical testing and operation of the drive after installation, it is important the HGA be similarly constrained during both functions.
During operation of the drive, it is optimal for the HGA's base plate to be pulled down with approximately three-to-seven pounds of force relative to a reference surface and fastened by swaging a boss hole in the HGA to a rotary arm in the drive. In the past, the HGA's base plate has been held by attaching the HGA to an intermediate mounting fixture and then placing the intermediate mounting fixture on the tester. This may require manually screwing the HGA to the intermediate mounting fixture before placing it on the tester. While this is an accurate method of mounting and provides the needed downward force, it is very labor intensive and adds an extra amount of error contributed by the fixture to the testing process.
Another method of attaching the base plate to an intermediate mounting fixture involves using a flexure clamp jaw that clamps the HGA's boss hole. While this is less labor intensive than manually screwing the HGA in place, it still requires extensive manual handling of the part. It also does not provide any downward force, which leaves the base plate unconstrained and not flat. This negatively affects the test results.
Still another method includes mounting the HGA to an intermediate mounting fixture that holds the HGA by pinching it with a clamp between the back of the base plate and a pin through a swage hole present in the HGA. This method also does not provide sufficient downward force, but forces the back edge of the base plate to align to the clamp. Because the back edge of the base plate is not a controlled edge, this may cause misalignment during the testing process.
All of the above-described methods are difficult to automate and have costs associated with loading, purchasing, and maintaining the extra fixtures on which the HGAs are mounted. The intermediate mounting fixtures also create a larger mass and require an additional mechanical interface, both of which create another potential source of error or vibration during the dynamic electric test.
In current systems, pins on the intermediate mounting fixture align the interconnection pads on the HGA's flex circuit with the dynamic electric tester's preamplifier contacts so that interconnection between the two is achieved. This alignment is necessary because the flex circuit is flexible, which permits the location of the interconnection pads to vary over a relatively wide area. After the flex circuit is constrained for interconnection, the intermediate mounting fixture, which holds the HGA, is loaded onto the tester.
The set of holes and pins used to align the intermediate mounting fixture for testing the HGA affects Reader Writer Offset (RWO), which is an important measurement in the dynamic electrical test. RWO is the distance a read/write head jogs to read a track that it has just written. The RWO is a function of the skew angle of the head, which is the angle of the head relative to the center of the disc on the x-y plane, the reader-to-writer separation distance, and the reader-to-writer alignment on the head. Because RWO data is used to verify that the reader-to-writer separation and alignment are within the tolerance limits of the process controls, the process of loading the HGA to disc should be carefully controlled so that the position of the HGA's skew angle is both accurate and repeatable.
Challenges to accurately loading an HGA to a disc include maintaining the HGA's orientation precisely from the moment it is put on the load mechanism until it is loaded to the disc. Also, one must ensure that the structure stiffness of the load mechanism does not contribute to positional error during test. Additionally, the process of loading the HGA must be carefully controlled to prevent damage to the HGA. For instance, the HGA cannot be bent significantly beyond its normal operating state. It also must be presented to the disc at a shallow enough angle to prevent any features from unintentionally contacting the disc during loading.
The cost effectiveness of the loader is not only measured in direct hardware costs due to damaged parts, but also in its cost effectiveness on the testing process. For instance, costs associated with the loader include the down time for the tester when there are changes in product configuration. Another cost includes the cost of testing media, which is one of the greatest costs in HGA testing. HGAs can crash a disc for several reasons during test including contamination of the media, non-optimal load orientation of the HGA, and HGAs with extreme or out-of-specification roll or pitch values.
Currently, load mechanisms typically include vertical translating stages, ramp loads, or tilt mechanisms. The vertical translating stage maintains the base plate of the HGA parallel to the disc and lifts the HGA to the disc. New generations of HGAs have features on the load beam beyond the head that contact the disc before the head and can damage the disc. This can result in a limited number of loads before the HGA or media are crashed. The ramp load mechanism works well in the drive, but it is difficult to use in the testing process. The ramp is typically fixed in location at the parameter of the disc, which limits the loading of the HGA to only one radius. Once that radius is crashed, that disc must be discarded. Ramp loading also can result in damage to the HGA from the sliding action across the ramp if the appropriate materials or surface finish are not used. However, use of the ramp enables loading the HGA to the disc at a shallower angle.
The third and often used loading method utilizes a tilting mechanism. A stage pivots, lowering the HGA below the surface of the disc. Once the HGA is moved into position under the disc surface, the HGA is pivoted up to the disc surface. One of the challenges of this mechanism is where to locate the hinge. The ideal location of the pivot point is near the bend in the HGA. The hinge, however, needs to be in real space and cannot inhibit positioning the HGA at various places on the disc. Miniaturized loaders with small pivot bearings have been used, but it is difficult to achieve the required structural stiffness and maintain all the tight tolerance required with a small structure and still provide room to access the HGA with an electrical interconnection. Though the HGA can be loaded at an angle shallower than the vertical load, it does not sufficiently reduce stresses to the HGA during load.
Once the HGA's head is loaded to the disc, there are still many sources of positional disturbance that can affect the effective track density during testing. For instance, disc flutter is a result of exciting a disc at the disc's natural resonant frequencies. Internal and external sources, such as spindle motor vibrations or external air turbulence and acoustic vibrations, may create vibrations that excite a disc and create disc flutter. The flutter is primarily a vertical modulation of the disc while the disc is rotating. The modulation creates bends in the disc. The compliance in the HGA load beam allows the head to follow undulations of the disk, but because the base plate of the HGA is held fixed relative to an external reference there may be a small error. This error is significant at current and higher track densities. The radial motion contributes to the total asynchronous runout, which exists when position errors are asynchronous, or do not repeat on each disc revolution.
Some current systems use devices between the spindle motor and the disc to reduce flutter. In these systems, one must test the HGA's head on the disc side that is opposite of these devices. Common approaches either require testing the top surface of the disc, which necessitates that the spindle protrude down into the test stand, or inverting the spindle that holds the disc so that the testing can be performed on the bottom surface of the disc. Both approaches have disadvantages. Systems that test the top surface are at a disadvantage from a part handling standpoint because testing on the bottom surface is considered more compatible with how the HGA is presented to the HGA tester. Additionally, placing a disc on the spindle is more difficult in current systems that test the top surface because of the close proximity of the flutter reduction device to the seated disc. Current testers may need extra mechanisms, such as guide fingers, to guide the disc to its final position. Current systems that test the bottom surface of the disc also have disadvantages because they typically invert the spindle, which creates a number of structure challenges in order to maintain the required rigidity and access for disc changes and other service needs.
During dynamic electrical testing, the intermediate HGA mounting fixture also has a large effect on the test and the test results. The intermediate mounting fixture can add to the stack-up tolerance related to the HGA's z-height causing small shifts in fly height. By increasing the mass that the tester micro-positioner must move while testing, the presence of the intermediate mounting fixture can lead to lower dynamic electrical tester track per inch (TPI) capability. The size of the intermediate mounting fixture also can limit the radii and skew locations that the HGA is loaded onto and unloaded from the disc. By limiting the load radii and skew options, the tester may use more media and take more time to load the HGA, which decreases the number of HGAs tested in a given period of time.